Grounding in electrical wiring systems is done primarily as a safety measure and, in such wiring systems, one of the current carrying conductors is connected to ground, to provide a system ground or neutral, and a separate ground wire or metallic conductor is provided as an equipment ground. It is common practice to use polarized plugs and receptacles to connect portable tools and appliances and mobile units to such a grounded wiring system. The use of polarized plugs and receptacles, if they are properly wired, should assure that the voltage applied to the portable appliance or tool or mobile unit was of the proper polarity. However, errors are sometimes made in connecting the wires at some point up to the power supply receptacle, or in connecting the wires of an extension cord to either the plug or receptacle at opposite ends of the extension cord, and such errors in wiring can cause the application of an incorrect or reverse polarity to the portable tool or appliance or mobile unit, with consequent safety hazards.
Some mobile units such as mobile homes and trailers have three-wire plus ground type internal wiring systems in which one wire is the grounded neutral and two other wires are hot wires. However, some trailer parks and the like only provide two-wire plus ground service and difficulties are encountered in connecting a mobile unit having a three-wire plus ground system to a two-wire plus ground service to provide adequate power to the mobile unit.
It has heretofore been proposed, for example as shown in U.S. Pat. Nos. 3,626,354; 3,733,576; and 3,851,243 to provide an adapter for connecting a grounded power supply to a portable tool or appliance, and which included some means for indicating when the polarity was reversed and an arrangement in which either the plug or the receptacle end of the adapter could be manipulated for connection in either of two different positions to a complementary polarized connector to reverse the polarity between the power supply and the portable tool or appliance. Such adapters, however, required, manual disconnection of either the plug or receptacle end of the adapter from the system accompanied by manual reversal of the position of the adapter and reconnection of the adapter in the system. Moreover, such adapters were not suitable for connecting a three-wire plus ground load system to a two-wire plus ground power supply.